This invention relates to the field of transporting and storing spent nuclear fuel and specifically to transferring spent nuclear fuel from a spent nuclear fuel pool to a storage cask.
In the operation of nuclear reactors, it is customary to remove fuel assemblies after their energy has been depleted down to a predetermined level. In the commercial nuclear industry, fuel assemblies are typically an assemblage of long, hollow, zircaloy tubes filled with enriched uranium. Upon depletion and subsequent removal, spent nuclear fuel is still highly radioactive and produces considerable heat, requiring that great care be taken in its packaging, transporting, and storing. Specifically, spent nuclear fuel emits extremely dangerous neutrons and gamma photons. It is imperative that these neutrons and gamma photons be contained at all times.
In defueling a nuclear reactor, the spent nuclear fuel is removed from the reactor and placed in a canister that is submerged in a spent nuclear fuel pool. The pool facilitates cooling of the spent nuclear fuel and provides radiation shielding in addition to that which is supplied by the canister. However, the canister alone does not provide adequate containment of the radiation. As such, a loaded canister cannot be removed or transported from the spent nuclear fuel pool without additional radiation shielding. Because it is preferable to store spent nuclear fuel in a xe2x80x9cdry state,xe2x80x9d the canister must eventually be removed from the spent nuclear fuel pool. As such, apparatus that provide additional radiation shielding during the transport and long-term storage of the spent nuclear fuel are necessary.
In state of the art facilities, this additional radiation shielding is achieved by placing the loaded canisters in large cylindrical containers called casks. There are two types of casks used in the industry today, storage casks and transfer casks. A transfer cask is used to transport canisters of spent nuclear fuel from location to location while a storage cask is used to store spent nuclear fuel in the xe2x80x9cdry statexe2x80x9d for long periods of time. Both transfer casks and storage casks have a cavity adapted to receive a canister of spent nuclear fuel and are designed to shield the environment from the radiation emitted by the spent nuclear fuel.
Storage casks are designed to be large, heavy structures made of steel, lead, concrete and an environmentally suitable hydrogenous material. However, because the focus in designing a storage cask is to provide adequate radiation shielding for the long-term storage of spent nuclear fuel, size and weight are often secondary considerations (if considered at all). As a result, the weight and size of storage casks often cause problems associated with lifting and handling.
Typically, storage casks weigh approximately 150 tons and have a height greater than 15 ft. As such, a common problem associated with storage casks is that they are too heavy to be lifted by most nuclear power plant cranes. Another common problem is that storage casks are too large to be placed in spent nuclear fuel pools. Thus, in order to store a canister of spent nuclear fuel in a storage cask, the canister must be removed from the pool, prepared in a staging area, and transported to the storage cask. Adequate radiation shielding is needed throughout all stages of this transfer procedure.
Removal from the storage pool and transport of the loaded canister to the storage cask is facilitated by a transfer cask. In facilities utilizing transfer casks to transport loaded canisters, an empty canister is placed into the cavity of an open transfer cask. The canister and transfer cask are then submerged in the storage pool. As each assembly of spent nuclear fuel is depleted, it is removed from the reactor and lowered into the storage pool and placed in the submerged canister (which is within the transfer cask). The loaded canister is then fitted with its lid, enclosing the spent nuclear fuel and water from the pool within. The canister and transfer cask are then removed from the pool by a crane and set down in a staging area to prepare the spent nuclear fuel for storage in the xe2x80x9cdry state.xe2x80x9d Once in the staging area, the water contained in the canister is pumped out of the canister. This is called dewatering. Once dewatered, the spent nuclear fuel is dried using a suitable process such as vacuum drying. Once dry, the canister is back-filled with an inert gas such as helium. The canister is then sealed and the canister and the transfer cask are once again lifted by the plant""s crane and transported to an open storage cask. The transfer cask is then placed atop the storage cask and the canister is lowered into the storage cask.
Because it is imperative that the loaded canister is not directly exposed to the environment during the step of lowering the canister from the transfer cask into the storage cask, transfer casks have bottoms that can be withdrawn so that the canister can be lowered directly into the storage cask. In prior art transfer casks, a rectangular compartment is attached to the bottom of the transfer cask. Within this rectangular compartment are two retractable sliding plates. When closed, these retractable plates act as the floor of the transfer cask""s cavity on which the loaded canister rests. When fully retracted, the retractable plates leave an unobstructed path leading from the transfer cask to the storage cask through which the canister can be lowered. While the retractable plates and rectangular compartment provide radiation shielding for the canister as it passes between the transfer cask and the storage cask, this transfer cask design and transfer procedure have a number of deficiencies.
First off, it should be noted that the external surface of a loaded canister is in continuous contact with the ambient air after it is placed in a storage cask. Thus, it is desirable that the external surface of the canister remain free of any radioactive contamination. However, because it is virtually impossible to seal the retractable plates because of the hardware (rollers, tracks, etc.) required to make the plates retractable, the retractable plates of prior art transfer casks are ineffective in preventing the intrusion of pool water (which may contain radioactive particulates in emulsion) into the space between the canister""s external surface and the walls of the transfer cask cavity. As such, the external surface of the canister can become contaminated. In order to deal with this threat of contamination, power plants employ a variety of measures such as continuously flushing the space with clean water from an external source. Such measures greatly complicate the process of fuel loading in the pool, leading to additional fuel loading time, added cost, and added risk to the operations staff who must work above the pool.
Second, as mentioned above, the transfer of the canister from the transfer cask to the storage cask occurs in a configuration where the transfer cask is stacked atop the storage cask. Because of the size of the transfer cask and storage cask, this stack can be quite tall, reaching heights of over thirty-five feet. Therefore, physical stability is a matter of concern, especially if a seismic event were to occur. As such, it is preferable to secure the transfer cask and the storage cask together to make the stack more robust. However, the presence of the retractable plate assembly at the bottom of the transfer cask precludes the design opportunity to configure a fastening detail. As a result, prior art transfer cask designs result in the undesirable situation where the transfer cask and the storage cask are stacked without being physically unconnected to each other.
Third, the retractable door assembly (including the retractable plates and the rectangular compartment) is quite heavy, reaching weights in excess of 12,000 lbs. As such, the area where radiation shielding is most needed, namely the cylindrical body of the transfer cask, must be made lighter to accommodate the heavy bottom region in order to remain within the lifting capacity of the power plant crane. Because the amount of radiation shielding provided by the transfer cask""s cylindrical body is directly proportional to its weight, the heavy retractable door assembly results in a reduced amount of radiation shielding.
Fourth, the hardware of the retractable door assembly, such as the rollers and tracks, require lubricant or grease to work properly. Submersing this lubricant in the pool can result in the undesirable result of contaminating the pool water.
Finally, prior art transfer cask designs utilizing the retractable door assembly may not fit into the spent fuel pools of some nuclear power plants. This problem results because the rectangular compartments often have a large footprint which is necessitated by the presence of the retractable plates.
These and other problems are solved by the present invention which in one aspect is an apparatus for use in transferring a canister of spent nuclear fuel from a transfer cask to a storage cask, the transfer cask having a bottom surface, a bottom lid adapted to be secured to and unfastened from the bottom surface, and a cavity adapted for receiving the canister, the storage cask having a top surface and a cavity adapted for receiving the canister, the apparatus comprising: a radiation absorbing shield surrounding a portion of a hole through which the canister can pass; means for securing the apparatus to the top surface of the storage cask; means for securing the bottom surface of the transfer cask to the apparatus; wherein the transfer cask securing means and the storage cask securing means are positioned on the apparatus so that when the apparatus is secured to both the transfer cask and the storage cask, the cavity of the transfer cask, the hole, and the cavity of the storage cask are substantially aligned; and means for moving the bottom lid in a horizontal direction once the bottom lid is unfastened from the bottom surface.
It is preferable that the horizontal moving means be adapted to move the bottom lid between an open and closed position. When the horizontal moving means is in the open position, an unobstructed path is formed between the cavity of the transfer cask, through the hole of the mating apparatus, and into the cavity of the storage cask. When in the closed position, the horizontal moving means is in a position to receive the bottom lid of the transfer cask. Preferably, when the horizontal moving means receives the bottom lid and moves the bottom lid to the open position, the bottom lid together with the radiation absorbing shield substantially surround and enclose the hole, the hole being unobstructed. Also preferably, the bottom lid is circular and the radiation absorbing shield is U-shaped comprising a semi-circular portion and a pair of substantially parallel legs, the diameter of the bottom lid being substantially equal to a perpendicular distance between the legs. The horizontal moving means can comprise a slidable tray and the radiation absorbing shield can comprise low friction tracks on which the slidable tray may slide.
Preferably, the apparatus further comprises a top plate and bottom plate, the top and bottom plates having an opening through which the canister can pass, the openings substantially aligned with the hole. In this embodiment, the horizontal moving means comprises a slidable tray and the bottom plate comprises low friction tracks on which the slidable tray may slide.
Moreover, it is preferable for the apparatus to further comprise means for lowering the bottom lid of the transfer cask in a controlled manner onto the horizontal moving means when the transfer cask is secured to the apparatus and the bottom lid is unfastened. These lowering means can be one or more pneumatic or hydraulic lifters and can be located directly on the horizontal moving means.
The apparatus""s means for securing the apparatus to the storage cask can be a plurality of bolt holes wherein the apparatus is secured to the top surface of the storage cask by extending bolts through the plurality of bolt holes and threadily engaging threaded holes located on the top surface of the storage cask. Additionally, the apparatus""s means for securing the transfer cask can be a plurality of threaded holes, the transfer cask being secured to the apparatus by extending bolts through holes located on the bottom surface of the transfer cask and theadily engaging the plurality of threaded holes of the apparatus.
Preferably, the radiation absorbing shield is substantially U-shaped and is constructed of concrete or lead. Also, preferably, the means for securing the apparatus to the storage cask and the means for securing the apparatus to the storage cask are positioned on the apparatus so that the apparatus can be secured to and unfastened from both the transfer cask and storage cask simultaneously.
In another aspect, the invention is a transfer cask for transporting a canister of spent nuclear fuel from a spent nuclear fuel pool to a storage cask comprising a cylindrical body having a top surface, a bottom surface, and a cavity adapted for receiving the canister, the bottom surface comprising means for securing and unfastening a bottom lid and means for securing to a mating device, the top surface comprising means to secure a cask lid; a bottom lid, the bottom lid acting as a floor for the cavity when secured to the bottom surface; a cask lid; a bottom seal positioned between the bottom lid and the bottom surface; and an annulus seal at or near the top surface of the cylindrical body and positioned between the canister and the cylindrical body when the canister is resting in the cavity.
Preferably, when the bottom lid is secured to the bottom surface, a hermetic seal is formed. It is also preferable that the means for securing the bottom lid, and means for securing the mating device, be positioned on the bottom surface so that the bottom lid can be unfastened and removed from the bottom surface while the transfer cask is secured to a mating device. If the bottom lid and bottom surface are circular, this can be accomplished by the circumference of the circular bottom being smaller than the circumference of the bottom surface.
Also, preferably, the bottom surface of the transfer cask is formed by a bottom flange. In this embodiment, the means for securing the bottom lid can be a plurality of bottom lid bolt holes wherein the bottom lid would comprise a plurality of threaded holes, the circular bottom lid being secured to the bottom flange by extending bolts through the bottom lid bolt holes and threadily engaging the threaded-holes of the circular bottom lid. Additionally with respect to this embodiment, the means for securing to a mating device can be a plurality of mating device connection holes, the transfer cask being secured to a mating device by extending bolts through the mating device connection holes of the bottom flange and threadily engaging threaded holes located on the mating device. The bottom flange can be circular having an outer perimeter wherein the means for securing to the mating device are closer to the outer perimeter than the means for securing the bottom lid.
The bottom seal can be a gasket fitted in a groove on the bottom lid. Moreover, the annulus seal can be a circular gasket.
In yet another aspect, the invention is a system for transferring spent nuclear fuel from a spent nuclear fuel pool to a storage cask comprising a fuel canister, a transfer cask, a storage cask, and an apparatus as described above; the storage cask comprising a top surface, means for securing the apparatus, and a cavity adapted for receiving the canister; the transfer cask comprising a bottom surface, a bottom lid adapted to be secured and unfastened to the bottom surface, means for securing to the apparatus, and a cavity adapted for containing the canister.
In regards to the system, it is preferable that the transfer cask comprise a bottom seal positioned between the bottom lid and the bottom surface and an annulus seal positioned between the canister and the transfer cask when the canister is contained in the transfer cask cavity. It is also preferable that the apparatus""s transfer cask securing means and storage cask securing means are positioned on the apparatus so that the apparatus can be secured to and unfastened from the transfer cask and storage cask simultaneously. Finally, the transfer cask""s means for securing to the apparatus are preferably positioned on the transfer cask so that the bottom lid can be unfastened and removed from the bottom surface while the transfer cask is secured to the apparatus.
In still another aspect, the invention is a method for transferring spent nuclear fuel from a reactor to a storage cask comprising submersing a transfer cask having a removable bottom lid and a cavity containing a canister into a spent nuclear fuel pool; placing spent nuclear fuel in the canister; securing the apparatus of claim 1 to a storage cask having a cavity adapted for receiving the canister; removing the transfer cask from the pool and securing the transfer cask to the apparatus; unfastening the bottom lid and horizontally moving the bottom lid with the apparatus;
and lowering the canister from the transfer cask into the cavity of the storage cask. It is preferable that this method further include the steps of securing a lid to the canister after placing the spent nuclear fuel in the canister; placing the transfer cask down in a staging area and preparing the canister for dry storage; and securing a cask lid to the transfer cask.